The present invention relates generally to butt welding machines and more particularly to machines for butt welding together a pair of laterally spaced apart, parallel, longitudinal edge portions such as are found at the longitudinal seam of an automotive wheel rim blank formed from a strip of metal which has been rolled into a cylinder.
Typically, the longitudinal edge portions of a rim blank are clamped between the inside and outside jaws of a respective one of a pair of laterally spaced clamping assemblies located on opposite sides of a weld zone which extends along the rim blank's longitudinal seam. When the rim blank is first clamped, its longitudinal edges are spaced laterally from each other. The clamped edges are then pressed laterally together along the longitudinal seam and, while lateral pressure is being applied to the edges, an electric welding current is passed through the clamping assemblies and the rim blank to butt weld the longitudinal edges to one another as they undergo an upsetting deformation.
A butt welding machine for performing the operation described in the preceding paragraph is described in Tan, et al., U.S. Pat. No. 4,562,326 ('326) entitled "Machine for Welding Automotive Wheel Rim Blanks", and the disclosure thereof is incorporated herein by reference.
In the butt welding machine of Tan, et al. '326, the rim blank is supported by a pair of horizontally extending, cantilevered horn members, each having an unsupported end. A stationary inner or upper jaw is located on the bottom of each cantilevered horn member. This upper jaw has an engaging surface for engaging the inner or upper surface of one of the longitudinal edge portions which undergoes welding. The upper jaw has first and second jaw ends, and the engaging surface of the jaw extends between the first and second jaw ends in the same direction as the cantilevered horn member. The machine also includes an outer or lower jaw for clampingly mating with each of the two upper jaws. Each lower jaw engages the outer or lower surface of a longitudinal edge portion which undergoes welding.
There is a vertically disposed slide member mounting each lower jaw for sliding, vertical movement toward and away from a corresponding upper jaw in a clamping assembly, to effectuate the clamping operation. In addition, one of the two cantilevered horn members and the slide member associated with that horn member are mounted for pivotal movement about a horizontal axis to urge the two jaws in that clamping assembly laterally toward the two jaws in the other clamping assembly, to effectuate the lateral pressure on the two longitudinal edges undergoing butt welding.
Because each horn member is cantilevered and because the upward movement of a slide member exerts an upward pressure on the cantilevered horn member all the way out to its unsupported end, there is a tendency for the unsupported end to be deflected upwardly relative to the fixed end of the cantilevered horn member. The upper jaw carried by the horn member is similarly deflected, with the horn member.
Each longitudinal edge portion of a rim blank is clamped between upper and lower jaws during welding. As a result of the deflection described above, there can be an inequality in clamping pressure along the longitudinal edge portion, from the front end to the rear end thereof. This is undesirable because such inequalities in clamping pressure produce inequalities in welding current from the front end to the rear end of the longitudinal edge portion, whereas a uniform distribution of welding current is desired.
The machine of Tan, et al. '326 employs a screw-down arrangement to try to cope with this problem. Although that screw-down arrangement helps to overcome the problem of non-uniform welding current distribution, it would be desirable to employ a solution other than a screw-down arrangement.
There may be times when it is desirable to exert a non-uniform clamping pressure along a rim blank's longitudinal edge portion from the front end to the rear end thereof. This would occur when there are surface irregularities, the adverse effects of which can be overcome by applying non-uniform pressure at localized spots along the length of the longitudinal edge portion.
Each cantilevered horn member is located near the front of the machine and extends in a frontward direction from the front end of a massive mounting member having a rear end located near the rear of the machine. In the machine of Tan, et al. '326, welding current is supplied to each of the upper jaws on a respective cantilevered horn member, by bus bars each extending the length of a horn member and its respective mounting member. Current flows from the rear end of one mounting member to the unsupported end of the corresponding horn member, through the corresponding upper jaw, through the rim blank, across the seam undergoing welding, through the upper jaw on the other horn member and then back through the other horn member from its unsupported end to the rear end of the associated mounting member. As a result, current flows in two opposite, parallel directions, through the respective horn members and associated mounting members. This produces mutually repelling magnetic fields around each of the two horn members and associated mounting members, and the two cantilevered horn members and their associated mounting members tend to repel each other laterally. This is in contrast to what occurs during a butt welding operation in which one of the two cantilevered horn members and its mounting member are urged in a lateral direction toward the other horn member and mounting member by a lateral force exerted primarily at the front end of the machine. The end result is that, during a butt welding operation, the one horn member and its mounting member are subjected to a horizontal bending force along the length thereof, and this is undesirable because the bending force is exerted at least in part along the length of the upper jaw on that horn member, from the front end to the rear end of the jaw.
In a butt welding operation in accordance with Tan, et al. '326, the rim blank is mounted around the two cantilevered horn members using a loading tool which is inserted into the seam between the two longitudinal edge portions of the rim blank. This enables accurate alignment and initial lateral spacing of the two longitudinal edge portions within and between the clamping assemblies. To facilitate the employment of the loading tool and to assure that each clamping assembly is aligned with a respective longitudinal edge portion at the spacing defined by the loading tool, it is desirable that each clamping assembly be initially spaced apart laterally from the other clamping assembly by a predetermined, fixed distance. Then, after both longitudinal edge portions of the rim blank have been clampingly engaged, it is desirable that the two clamping assemblies be spaced apart laterally a greater, predetermined, fixed distance, to facilitate the removal of the loading tool; however, this latter distance must not be too great, to avoid adversely deforming the rim blank by spreading it apart at the seam.